How cells follow DNA directions on how to make proteins Protein Synthesis How cells follow DNA directions on how to make proteins

DNA = the directions in code Each chain of nucleotides is a line of code Each chain contains directions for making many different proteins Each set of directions is a gene 1 gene = 1 protein =1 trait

Steps in making a protein 1) Copy the Directions for one protein onto RNA (one gene) Transcription 2) Use the RNA directions to make a protein Translation

1) Transcription A) RNA polymerase scans the DNA to find the gene it needs to copy B) RNA polymerase has 4 jobs 1) unwind the DNA double helix 2) breaks hydrogen bonds to separate chains 3) match RNA nucleotides to the DNA gene 4) fuse the RNA nucleotides together to make a chain of RNA http://www.youtube.com/watch?v=ztPkv7wc3yU&feature=related http://www.youtube.com/watch?v=5MfSYnItYvg&feature=related

The chain of RNA that is made is called mRNA Or messenger RNA

Messenger RNA Carries coded directions out into the cell. Where mRNA binds to a ribosome.

2) Translation A) Translates nucleotide code of RNA into the amino acid code of a protein B) Takes place in the cytoplasm C) Is done by a ribosome http://www.biostudio.com/demo_freeman_protein_synthesis.htm

mRNA mRNA = chain of nuclotides that make up the coded directions for making the protein Every 3 nuclotides of mRNA is a codon Each codon is the code for 1 amino acid

Ribosomes Made of 2 subunits. Subunits bind onto mRNA chain at start codon 3 codons at a time are inside the ribosome E is for Exit Codons in P and A sites are the ones being read

tRNA bring amino acids to the ribosome Match the correct amino acid in place by matching their anti-codon to the mRNA codon Fit into the E, P and A sites

tRNA Each tRNA only picks up ONE kind of amino acid tRNA drops off amino acids at the ribosome then moves back into the cytoplasm to pick up another amino acid(but always the same kind) aminoacyl-tRNA synthetase enzymes bind aa to tRNA

Steps to Translation A) Initiaion 1) Ribosome subunits bond to mRNA 2) tRNA that matches start codon binds at site P

3) tRNA that matches 2nd codon binds at site A B) Elongation 3) tRNA that matches 2nd codon binds at site A 4) ribosome attaches the 2 amino acids those tRNAs carry to each other by a peptide bond http://www.phschool.com/science/biology_place/biocoach/translation/elong1.html

wobble effect The last base in the anti-codon has some flexibility in what it binds to (wobble room) So anticodon AGU could bind to UCA or UCG

5) Ribosome moves down mRNA chain by one codon 6) the first tRNA Moves into the E site and exits the ribosome

7) the next tRNA moves into place

Termination 8) Ribosome moves down mRNA to stop codon 9) Release factors bind to stop codon instead of a tRNA 10) Ribosome, mRNA, tRNAs and protein all break apart

Mutations= changes in a cell’s DNA Caused by: 1) spontaneous errors 2) mutagens: chemicals/radiation Can happen in body cells (somatic cells) or In reproductive cells (egg & sperm)

Somatic cell mutations May cause no change = if mutated gene is one that is not use by that particular cell May cause cell to die May cause cancer Not passed on to children

Thymine dimer

Reproductive Cell or Early Embryo Cell Mutations Child with mutation Every cell of the child is mutated May cause miscarriage May cause a genetic disorder in child May have no effect at all

2 types of mutation Gene mutation = DNA coding error may be a missense mutation where the codons code for the wrong aa …. or nonsense where stop codon or partial codon Chromosome Mutation = change in chromosome number, missing or extra chromosome pieces.

Gene Mutations 1) point mutations (substitutions) – change only 1 base pair a) substitution of 1 nucleotide for another b) ATT becomes ATG..(missense) c) may not cause any change = silent mutation d) may change 1 amino acid e) could be nonsense only if codes for stop e) can cause disorder

2) Frameshift mutations- change all the codons a) insertion b) deletion c) THE FAT CAT = HEF ATC AT d) most always ruins protein

3) tandem repeats – codons repeated over & over most often ruins protein more repeats = more problems

Transcripts Transcript = chain of RNA as copied from DNA template Pre-mRNA = a transcript that will become mRNA Other transcripts become tRNA or rRNA

Transcript processing Pre-mRNA transcript has a cap added to 5’ end cap facilitates exit through nuclear pore cap aids in translation initiation cap is added to a short UTR on 5’ end of pre-mRNA transcript (untranslated region) UTR is followed by the start codon

Poly-A tail 3’ end of transcript gets a poly-A tail added to it Enzyme adds 50 -250 more A ribonucleotides More As added make the mRNA last longer Fewer As make the mRNA break down quicker Breakdown starts immediately on entry to cytosol Hydrolytic enzymes

RNA splicing Introns = in between coding regions of RNA do not code for a.a.s are cut out before mRNA leaves nucleus bacteria do NOT have introns Exons = have codons that are executed by translation….code for a.a.s Introns are cut out, exons fused together

Alternative splicing Exons are spliced together in different orders To make different proteins

spliceosome Complex of proteins and small RNAs Remove introns Joins exons in proper order RNAs in spliceosome are Ribozymes… RNA enzymes Sometimes the intron being removed IS the ribozyme

Evidence for RNA before DNA RNA can act as its own enzyme RNA polymerase can initiate polymerization on its own DNA polymerase can only start polymerization at a primer That is created by RNA polymerase

Post-translational modification Amino acid modification – add functional groups Trim aa from cap end Cleave polypeptide into pieces Join 2 polypeptide into quaternary structure with disulfide bonds

Protein location w/i the cell All translation begins in cytoplasm For proteins destined for endomembrane system (ER, golgi, lysosome, plasma membrane) OR proteins produced for secretion (insulin) The first few aa at leading end of polypeptide = signal peptide Signal peptide binds to a Signal Recognition Particle (SRP) SRP escorts ribosome to receptor protein on ER